Field of the Invention
The present invention relates to a display device and a luminance control method therefore.
Discussion of the Related Art
Flat panel displays include a liquid crystal display device (LCD), a plasma display panel (PDP), an organic light emitting diode display (hereinafter, referred to as ‘OLED display’), an electrophoretic display device (EPD), etc. A liquid crystal display displays an image by controlling an electric field applied to liquid crystal molecules according to data voltages. An active matrix liquid crystal display has advantages of reduced prices and performance improvement with the development of the processing technology and the driving technology. Thus, the active matrix liquid crystal display is the most widely used display device applied to almost any display device, from small mobile device to large televisions.
Because the OLED display is a self-emitting device, it has lower power consumption and a thinner profile than a liquid crystal display requiring a backlight unit. Further, the organic light emitting display has advantages of wide viewing angle and fast response time. The OLED display is gaining market share while competing with liquid crystal displays.
Each pixel of the OLED display comprises an organic light emitting diode (hereinafter, referred to as ‘OLED’), which is a self-luminous element. As shown in FIG. 1, the OLED includes organic compound layers such as a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer HTL, and an electron injection layer EIL, which are stacked between an anode and a cathode. The OLED display reproduces an input image as the OLED of each pixel emits light when electrons and holes are combined in an organic layer by allowing current to flow through a fluorescent or phosphorescent organic thin film.
The OLED display may be classified into different types based upon the type of luminescence material, the emission scheme, the emission structure, the driving scheme, etc. The OLED display may be divided into fluorescent emission type and phosphorescent emission type according to the emission scheme, or divided into top emission type and bottom emission type according to the emission structure. Also, the OLED display may be divided into PMOLED (Passive Matrix OLED) and AMOLED (Active Matrix OLED) according to the driving scheme.
In order to efficiently reduce the power consumption of a display device, it is necessary to lower the luminance of the screen, which greatly affects electricity consumption. However, simply reducing luminance can reduce power consumption, but may result in picture quality degradation. For example, if the user decreases the luminance of display images, the luminance of a bright image with a high average picture level (hereinafter, ‘APL’) may become excessively low. The APL is defined as the average luminance of the brightest color in 1-frame image data and expressed by Equation (1):
                              A          ⁢                                          ⁢          P          ⁢                                          ⁢                      L            ⁡                          (              %              )                                      =                                            SUM              ⁢                              {                                                      Max                    ⁡                                          (                                              R                        ,                        G                        ,                        B                                            )                                                        /                  255                                }                                                    The              ⁢                                                          ⁢              total              ⁢                                                          ⁢              number              ⁢                                                          ⁢              of              ⁢                                                          ⁢              pixels                                ×          100                                    Equation        ⁢                                  ⁢                  (          1          )                    where R is represents red data, G represents green data, and B represents blue data. Max(R,G,B) is the maximum values of R, G and B, and SUM {Max(R,G,B)} is the sum of the maximum values of R, G and B.
An image containing a large amount of bright pixel data has a high APL. On the other hand, an image containing a small amount of bright pixel data has a low APL. The peak white gray level of 8-bit pixel data is gray value 255.
As shown in FIG. 2, if approximately 25% of the pixels on the entire screen have the peak white gray level and the remaining pixels have the black gray level 0 (zero), the APL is 25%. On the contrary, if the pixels on the entire screen have the peak white gray level 255, the APL is 100%. Hereinbelow, the luminance at the APL of 25% is referred to as peak luminance, and the luminance at the APL of 100% is referred to as full white luminance.
Peak luminance is higher than full white luminance because it causes less load on the screen. In the OLED display, more current flows through the OLEDs of the pixels at peak luminance and they emit brighter light than at full white luminance. Peak luminance control (hereinafter, ‘PLC’) is a method of reducing power consumption by decreasing luminance with increasing APL, based on the PLC curve shown in FIG. 3. The PLC curve defines the maximum luminance of pixels. The pixels of a display panel emit light at a level equal to or below the maximum luminance defined by the PLC curve. On the PLC curve of FIG. 3, luminance versus APL is defined in such a way that the maximum luminance of the pixels increases with decreasing APL and decrease with increasing APL.
The PLC curve of FIG. 3 is expressed by Equation (2). The PLC curve can be equally divided by 8 PLC points. When the user adjusts luminance through a user interface (UI), k in Equation (2) is adjusted in proportion to the amount of luminance adjustment by the user and the luminance at the PLC points at all APLs is adjusted by a fixed percentage.Pi=Pi×k   Equation (2)where i=0, 1, 2, 3, 4, 5, 6, and 7.k is a luminance adjustment variable. k=1.00˜0.
P0 is the peak luminance, and Pi is the luminance at the i-th PLC point which is lower than the peak luminance.
The related art PLC is problematic in that the full white luminance and the contrast ratio become excessively low if the user decreases the luminance of a display device. FIG. 4 shows the luminance variations on the PLC curve when the luminance of an OLED display decreases to 90% (k=0.9), 80% (k=0.8), 65% (k=0.65), 30% (k=0.3), and 20% (k=0.2).
Referring to FIG. 4, the figures in the table are digital values for determining luminance. The higher the digital values, the higher the luminance of the pixels. The digital values may be transmitted to the timing controller of the display device through I2C communication. The following description will be given under the assumption that the digital values are luminance values.
The initial luminance at the PLC points may be set to P0=255, P1=225, P2=205, P3=185, P4=165, P5=145, P6=120, and P7=100.
When the user decreases the luminance of the OLED display to 90% (k=0.90), the luminance at the PLC points decreases to P=218, P1=192, P2=175, P3=158, P4=141, P5=124, P6=103, and P7=86 according to Equation (2). This means that the luminance of the OLED display decreases to 90% of the initial values at all APLs.
When the user decreases the luminance of the OLED display to 80% (k=0.80), the luminance at the PLC points decreases to P=184, P1=162, P2=148, P3=133, P4=119, P5=104, P6=86, and P7=72 according to Equation (2). This means that the luminance of the OLED display decreases to 80% of the initial values at all APLs.
According to the related PLC, when the user decreases the luminance of a display device, the luminance decreases by a fixed percentage at every APL. Thus, the full white luminance becomes excessively low, as indicated by the dotted circle in the graph of FIG. 4. Because most of the pixels on the screen are turned on, a significant decrease in full white luminance and a sharp decline in contrast ratio are observed. Accordingly, PLC control requires a solution to avoid excessive decreases in full white luminance.